This invention pertains to systems and methods for operating battery-powered implantable medical devices.
Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. Implantable cardioverter/defibrillators (ICDs) are devices that provide defibrillation therapy by delivering a shock pulse to the heart when a tachyarrhythmia such as fibrillation is detected and typically may also provide pacing therapy. An ICD is an electronic device containing a shock pulse generator that is usually implanted into the chest or abdominal wall. Electrodes connected by leads to the ICD are placed on the heart, or passed transvenously into the heart, to sense cardiac activity and to conduct the impulses from the pulse generator. Typically, the leads have electrically conductive coils along their length that act as electrodes. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, by delivering a shock pulse that impresses an electric field between the electrodes to which the pulse generator terminals are connected. ICDs typically use an electrolytic capacitor that is charged from a battery with an inductive boost converter to deliver the shock pulse. When ventricular fibrillation is detected, the ICD charges up the capacitor to a predetermined value for delivering a shock pulse of sufficient magnitude to convert the fibrillation (i.e., the defibrillation threshold). The capacitor is then connected to the shock electrodes disposed in the heart to deliver the shock pulse.
ICDs are powered by a battery contained within the housing of the device that has a limited life span. When the battery fails, it must be replaced which necessitates a reimplantation procedure. The useful life of the battery may vary in each individual case and depends upon the specific battery and the power requirements of the device. For example, a device which must deliver paces and/or defibrillation shocks on a frequent basis will shorten the useful life of the battery. As the battery depletes, it is desirable to provide a means of determining that the battery is near the end of its life so that replacement of the battery can be scheduled rather than done on an emergency basis.
The present invention is a method and apparatus for monitoring the condition of a battery in an implantable medical device such as an implantable cardioverter/defibrillator. In accordance with the invention, the open circuit voltage of the battery and the time required to charge an electrolytic energy storage capacitor to a specified full voltage value are both measured on an intermittent or periodic basis. The capacitor charge time measurement may be taken during periodic reforming of the electrolytic energy storage capacitor. The operating status of the battery may then be designated as being one of a plurality of battery states ranging from best to worst based upon those measurements. In one embodiment, a similar set of battery states are used to represent the open circuit voltage status and the capacitor charge time status of the battery. The battery operating status is then the worse of the open circuit voltage status and the capacitor charge time status. In order to detect deterioration in the battery""s condition in a more timely manner, the time interval for reforming the energy storage capacitor and measuring the capacitor charge time is adjusted based upon the present battery operating status.